Electric fields and Coulomb's lawEdexcel International A Level Physics: Subtopic test

1. 1
   Two very small charged spheres, A and B, carry charges of +2.0 nC and +5.0 nC respectively. They are held 4.0 cm apart in air, so they may be treated as point charges in a vacuum.

   (a)

   Which statement correctly describes the electrostatic forces between the two spheres?

   [1 mark]

   • AAttractive, and larger on B because B has the larger charge
   • BRepulsive, and equal in magnitude on each sphere
   • CAttractive, and equal in magnitude on each sphere
   • DRepulsive, and larger on B because B has the larger charge

   (b)

   The spheres are moved so that they are 8.0 cm apart. How does the force between them compare with its original value?

   [1 mark]

   • AHalf of the original value
   • BDouble the original value
   • COne quarter of the original value
   • DFour times the original value

   (c)

   Calculate the magnitude of the force between the spheres when they are 4.0 cm apart.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a simple model of a hydrogen atom, an electron moves in a circular orbit around a single proton. The separation of the two particles is 5.3 × 10⁻¹¹ m. The magnitude of the charge on each particle is 1.60 × 10⁻¹⁹ C.

   (a)

   What is the magnitude of the electric field strength at the position of the electron, due to the proton?

   [1 mark]

   • A27 N C⁻¹
   • B8.2 × 10⁻⁸ N C⁻¹
   • C1.4 × 10⁻⁹ N C⁻¹
   • D5.1 × 10¹¹ N C⁻¹

   (b)

   What is the direction of the electric field at the electron's position due to the proton?

   [1 mark]

   • AAway from the proton, along the line joining them
   • BTowards the proton, along the line joining them
   • CAlong the tangent to the electron's orbit
   • DPerpendicular to the line joining them, in the plane of the orbit

   (c)

   Calculate the force on the electron due to the proton and state its direction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A small metal sphere carries a charge of +6.0 nC. Point P is 0.30 m from the centre of the sphere, in air.

   (a)

   Calculate the electric field strength at P and state its direction.

   [3 marks]

   (b)

   A small charge of −4.0 nC is placed at P. Calculate the force on it and state the direction of this force. Determine the force on the same charge when it is moved to a point 0.60 m from the centre of the sphere.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two point charges are fixed in a vacuum. Charge A is +4.0 nC and charge B is +9.0 nC. They are 0.30 m apart, and the points discussed lie on the straight line joining them.

   (a)

   Show that there is a point between the charges where the resultant electric field strength is zero, and determine its distance from charge A. Explain why no such point exists outside the region between the charges.

   [6 marks]

   (b)

   A small test charge of +2.0 nC is placed at the midpoint of the line joining A and B. Calculate the resultant electric field strength at the midpoint and the force on the test charge, stating the direction of each.

   [6 marks]

   Total for question 4: 12 marks

End of questions